The behavior of GaN-based transistors as zero-bias detectors is very dependent on the configuration of the bias, the operation temperature and whether the radio-frequency power is fed in the drain or the gate terminal. In drain-injection, the negative current responsivity shows a bell-shape dependence on VGS for all the studied frequency range and temperatures. In the case of the voltage responsivity, depending on the temperature range an increase or decrease of the responsivity is observed in subthreshold conditions. For the gate-injection configuration, the voltage responsivity at low frequency is null only for VGS > V th . Surprisingly, in subthreshold conditions, it is very high and positive, contrary to the negative values expected for this configuration. The origin of this unexpected behavior in VGS < V th is that the drain terminal is self-biased at the zero-current operating point. An analytical model is able to explain the mechanism behind the observed dependencies of the experiments.